"Smoking alters adaptive immunity in a persistent way", concluded the authors of a study published on February 13 in the journal Nature.

The new study marks a significant progress in understanding the harmful effects of smoking on health, which, according to the World Health Organization (WHO), kills about 8 million people annually worldwide.

This brought attention to a previously neglected aspect: adaptive immunity, which develops as infections occur, stays compromised for several years after individuals stop smoking.

These conclusions are based on a sample of one thousand individuals. They were selected over 10 years ago, as part of a project coordinated by the Pasteur Institute in Paris, and their immunity has been regularly examined through various analyses, especially blood tests.

This type of project, called a cohort study, is highly robust in evaluating how different factors influence health and metabolism over time.

And, in this case, smoking has had the greatest influence, more than other factors, such as sleep duration and physical activity level, according to researchers led by biology specialist Violaine Saint-André.

However, the result is not entirely new. Doctors already knew that smoking affects innate immunity - common to all humans - by worsening inflammatory reactions.

The new study confirmed this, finding that this effect disappears immediately after quitting smoking. However, a significant revelation is that the situation is different for acquired immunity.

This continues to be impacted for certain individuals for years or even decades after they quit smoking, despite the sample size being too small and the reactions too variable to determine a precise average duration.

Researchers went even further, showing that these disruptions are associated with an "epigenetic" effect: The DNA of individuals stays constant, yet exposure to smoking influences the practical expression of specific genes.

It should by no means be concluded that quitting smoking serves no purpose. The negative effects ultimately disappear.

But "to maintain your immunity in the long term, it is better never to start smoking", emphasized Violaine Saint-André at a press conference.

The study, based on biological analyses, cannot, however, say what the consequences of these immune variations are on health.

According to its authors, there could be effects on the risk of infections, cancers or autoimmune diseases. But these are, at this stage, only hypotheses.

Another study, published in early February, attempted to determine to what extent health risks truly persist after a person quits smoking.

Published in NEJM Evidence, the study relied on medical data obtained from approximately 1.5 million people from Canada, the United States, Norway and the United Kingdom.

Its authors compared mortality among several groups: active smokers, people who have never smoked and former smokers.

For the latter, medical risks take time to completely disappear. Once a person quits smoking, they have to wait ten years to have a life expectancy comparable to that of a person who has never smoked.

However, even in this scenario, one should not infer that giving up smoking is not worthwhile. The study authors discovered that "benefits already appear after three years", with an average of five years of recovered survival in this group, representing half the journey towards a normal life expectancy.

And the beneficial effect is visible regardless of the age at which the person quits smoking, although it is more pronounced in individuals under 40 years old.